Connect edge with perpendicular edge
The fastest solution in this case may be using a knife tool. Press K and snap the knife to the vertices. After pressing the LMB press C to make the cut go straight and cut the longer piece horizontally. Repeat the process for the other vertex. Now you can use a bridge tool to connect pieces (and then dissolve the unnecessary edge). You may also use a snapping tool (vertex) to connect pieces and then remove doubles.